The present invention relates to x-ray imaging devices and, more particularly, to a device for producing digital x-ray images at high resolution.
A storage phosphor is a material, such as ZnSi:Cu or CaSO.sub.4 :Mn, which, when locally excited by high energy photons, enters a locally excited state which is stable indefinitely. The storage phosphor can be stimulated to return to its ground state, with the accompanying emission of phosphorescent light, by stimulation, for example by intense light of a wavelength different from that of the phosphorescence, or by imposing a voltage across the storage phosphor in excess of a certain threshold that depends on the thickness and composition of the storage phosphor. The intensity of the phosphorescence is proportional to the number of photons which originally were absorbed locally by the storage phosphor to put the storage phosphor into its excited state.
Storage phosphor layers have long been used to record latent x-ray images. The intensity of the phosphorescence of the storage phosphor layer is proportional to the intensity of the x-rays to which the layer was exposed to create the latent image. Typically, a storage phosphor layer is stimulated to phosphoresce by optical scanning. Representative patents in the field include U.S. Pat. No. 4,320,296 to Ishida et al., U.S. Pat. No. 4,816,679 to Sunigawa et al., U.S. Pat. No. 4,847,498 to Saito et al., U.S. Pat. No. 4,953,038 to Schiebel et al., U.S. Pat. No. 5,376,806 to Hejazi, and U.S. Pat. No. 5,654,556 to Yasuda.
To create a digital record of the x-ray image, the light emitted by the storage phosphor layer must be converted to an electronic signal. This conversion can be performed using a charge coupled device array (CCD). In the context of such arrays, the terms "detector" and "detector element" are used interchangeably herein to denote one element of such an array. These elements also are commonly called "pixels", but to avoid confusion, the term "pixel" is used herein only for elements of images. Although CCD arrays as large as about 9000.times.7000 elements are available, arrays larger than about 1000.times.1000 are very costly and require complicated electronics and sophisticated programming techniques for real-time imaging. The smaller, less costly arrays have too few detector elements to acquire images with enough pixels for medical imaging applications such as mammography.
There is thus a widely recognized need for, and it would be highly advantageous to have, a storage-phosphor-and CCD-based x-ray imaging device that records the x-ray image at a resolution higher than the resolution of the CCD array. Such a device would enable the recording of high resolution x-ray images using relatively small and inexpensive CCD arrays.